Metals such as platinum (Pt), palladium (Pd), ruthenium (Ru), and related alloys are known to be excellent catalysts. When incorporated in electrodes of an electrochemical device, such as a fuel cell, these materials function as electrocatalysts as the materials accelerate electrochemical reactions at electrode surfaces, yet are not themselves consumed by the overall reaction. Although noble metals have been shown to be some of the best electrocatalysts, their successful implementation in commercially available energy conversion devices may be limited due to high cost and scarcity. Noble metal catalysts may also be susceptible to carbon monoxide (CO) poisoning, poor stability under cyclic loading, and providing relatively slow conversion kinetics in oxygen reduction reactions (ORR).
A variety of approaches have been employed in attempts to address these issues. One approach involves increasing the overall surface area available for reaction by forming metal particles with nanometer-scale dimensions. Loading of more expensive noble metals such as Pt has been reduced by forming nanoparticles from alloys comprised of Pt and a low-cost component. Further improvements have been attained by forming core-shell nanoparticles, in which a core particle is coated with a shell of a different material that functions as the electrocatalyst. The core is usually a low-cost material which is easily fabricated whereas the shell comprises a more catalytically active noble metal. An example is provided by U.S. Pat. No. 6,670,301 to Adzic, et al., which discloses a process for depositing a thin film of Pt on dispersed Ru nanoparticles supported by carbon substrates. Another example is U.S. Pat. No. 7,691,780 to Adzic, et al. which discloses platinum- and platinum alloy-coated palladium and palladium alloy nanoparticle cores. Each of the aforementioned U.S. patents is incorporated by reference in its entirety as if fully set forth in this specification.
One approach for synthesizing core-shell particles with reduced noble metal loading and enhanced activity levels involves the use of electrochemical routes, which provide atomic-level control over the formation of uniform and conformal ultrathin coatings of the desired material on a large number of three-dimensional nanoparticles. One such method involves the initial deposition of an atomic monolayer of a metal such as copper (Cu) onto a plurality of nanoparticles by underpotential deposition (UPD). This is followed by galvanic displacement of the underlying Cu atoms by a noble metal such as Pt as disclosed, for example, in U.S. Pat. No. 7,704,918 to Adzic, et al. Another method involves hydrogen adsorption-induced deposition of a monolayer of metal atoms on noble metal particles as described, for example, by U.S. Pat. No. 7,507,495 to Wang, et al. Each of the aforementioned U.S. patents is incorporated by reference in its entirety as if fully set forth in this specification.
Core-shell particles having a core comprised of one or more non-noble metals may show gradual dissolution of the non-noble metal component over time. Exposure of the core to the corrosive environment typically present in energy conversion devices such as a proton exchange membrane fuel cell (PEMFC) due to, for example, an incomplete protective shell layer may result in gradual erosion of the non-noble metal components which leads to the loss of structural integrity of the particles. With continued operation, reduction in structural integrity may reduce the catalytic activity of the electrocatalyst and cause damage to the electrolyte membranes contained within a typical energy conversion device, thereby reducing its charge storage and energy conversion capabilities.
There is therefore a continuing need to develop catalysts with a still higher catalytic activity in combination with ever-lower loading of precious metals, enhanced durability, and long-term stability. Such catalysts should also be capable of being manufactured by large-scale and cost-effective processes suitable for commercial production and incorporation in conventional energy production devices.